For those who attended school in California, there is a better than even chance of being exposed to the fact that acorns were an important part of the diet of the Native Californians up to European colonization (and for many people, up through the early 20th century). When you begin looking into it, the reasons are pretty simple: the acorns are storeable (some species keep for up to a couple of years after being picked), predictable (you know where they'll be year after year), semi-stable (unlike many other food resources, when you go out looking for acorns, you will come back with some, even if some years were leaner than others), and very nutritious. However, while acorns are the emblematic food of California, they weren't used with any regularity until approximately 4,000 to 3,000 years ago in much of California, or 7,000 to 9,000 years after California was first populated (though there is evidence that acorns were used intensively as early as 8-9,000 years ago in some locations, especially in northwest California). Given the advantages of acorns this seems pretty strange, until you consider the cost of acorns in terms of time and labor.
Picking acorns, peeling/shelling them, pounding them into mush, and then leaching them of tannic acid is a long, laborious process. It can consume the efforts of an entire community, and can also lead to greater social rigidity due both to the semi-fixed locations for gathering and the social organization needed for efficient acorn gathering and processing. To put it in modern economic terms, I recently learned that a gallon of acorn mush made by members of a local tribe costs $50 - and that is after it is made using the more efficient technology of blenders and cheesecloth rather than stone mortars and woven baskets to process the acorn. So, we have a staple food, something that you would eat as part of your most basic diet, requiring so much work that with modern labor-saving devices it prices out at $50 a gallon.
Knowing that, it's fair to ask why people began eating acorns at all. Of course, like most things in archaeology, we don't know for certain, but, at least in California, we have some pretty good ideas.
The dominant idea regarding this, and I think it's generally a pretty sound one, is that the answer lies in a basic economic principle. Acorns are an expensive resource in that they take so much time, energy, and specific forms of social organization to gather and process, so it seems reasonable to expect that they would not be used if a cheaper (that is, more convenient) resource were available. Say, if something costs $10 per gallon as opposed to $50 a gallon. Now, I usually argue against over-extending these sorts of simplistic economic arguments - if they were as strong as many advocates of Adam Smith's writings believe then things such as designer jeans and urban SUVs would not exist - but they are useful if thought of as general trends rather than hard-and-fast laws of commerce. Proceeding this way, we can see that, while there are many, many exceptions, as a general rule it is true that when presented with two options for feeding a society, people tend to choose the easier/less expensive option* provided that it produces comparable nutrition value. In the case of acorns, there are numerous grass seeds, tubers, and other such basic foods that can serve much the same purpose that are significantly easier to gather and process, and in pre-acorn sites we see very few mortars but many, many milling basins and milling slicks of the sort that would be used for processing such plants. Over time, however, these milling tools, while never completely going away, begin to loose favor as compared to the mortar, which is good for processing oily seeds such as acorns.
There are alot of explanations for what happened, and given that acorns became the staple food at different times in different parts of California, I am of the opinion that several different things happened in several different places. In some places, the population may have grown sufficiently that it outstripped the capacity of the convenient resources, forcing people to use the more difficult ones. In other places, ecological change may have resulted in plant die-off that made the previously used resources more difficult to obtain or not sufficient to support the population. In still others, growing populations (and perhaps growing territorialism) may have resulted in less seasonal mobility and the need to use more local resources to make up for the fact that travel to resources was no longer feasible.
I could go on for a while, but I think you get the point - changes occurred which either made the previous staple foods more expensive in terms of time and labor involved in obtaining and processing them, or else rendered them insufficient for providing for the needs of the people depending on them. Generally the archaeological evidence seems to be consistent with this model - though it is one that is rather damnably difficult to test - but every new archaeologist looking at it teases out new and interesting elements. While most of us working in California would agree to the overall use of the economic model here, there are probably as many permutations of it as there are people looking at it.
*In case you're trying to think of an exception and stuck, then consider the number of foods that we eat that are actually quite expensive in terms of either money or time to prepare or obtain, but which we nonetheless consume whenever we get the chance because of intangibles such as taste or social prestige (Filet Mignon is a wildly impractical but delicious food, while I find it hard to imagine that anyone likes caviar enough to justify the price tag unless you take into account the social prestige that comes from having and serving caviar). Also, consider that many more difficult/expensive foods are consumed for reasons of ideology rather than reasons of economics, which explains the success of Whole Foods and similar stores. These types of behaviors all make sense, but only if you allow that economics is only one of many things that humans consider when making their choices.